2620
J. Phys. Chem. B 2004, 108, 2620-2630
Transient Isotopic Kinetic Study of the NO/H2/O2 (Lean de-NOx) Reaction on Pt/SiO2 and Pt/La-Ce-Mn-O Catalysts Costas N. Costa and Angelos M. Efstathiou* Department of Chemistry, UniVersity of Cyprus, P.O. Box 20537, CY 1678 Nicosia, Cyprus ReceiVed: July 30, 2003; In Final Form: NoVember 17, 2003
Steady-state isotopic transient kinetic analysis (SSITKA) coupled with temperature-programmed surface reaction (TPSR) methods and using in situ mass spectroscopy and DRIFTS have been applied for the first time to study essential mechanistic aspects of the NO/H2/O2 reaction at 140 °C under strongly oxidizing conditions over 0.1 wt % Pt/SiO2 and 0.1 wt % Pt/La-Ce-Mn-O catalysts. The nitrogen-pathway of the reaction from NO to form N2 and N2O gas products was probed by following the 14NO/H2/O2 f 15NO/H2/O2 isotopic switch at 1 bar total pressure. It was found that the chemical structure of active intermediate NOx species strongly depends on support chemical composition. In the case of the Pt/SiO2 catalyst, the reaction route for N2 and N2O formation passes through the interaction of one reversibly and one irreversibly NOx species chemisorbed on the Pt surface. On the other hand, in the case of a Pt/La-Ce-Mn-O catalyst, the reaction route passes through the interaction of two different in structure irreversibly chemisorbed NOx species on the support. For the latter catalyst, the mechanism of the reaction must involve a hydrogen-spillover process from the Pt metal to the support surface. A surface coverage θ ) 1.8 (based on Pt metal surface) of active NOx intermediate species was found for the Pt/La-Ce-Mn-O catalyst. A large fraction of it (81.5%) participates in the reaction path for N2 formation, whereas in the case of Pt/SiO2, this fraction was found to be 68.4% (active NOx, θ ) 0.65). These important results provide an explanation for the lower N2 reaction selectivity values observed on Pt/SiO2 compared to Pt/La-Ce-Mn-O catalyst. Inactive adsorbed NOx species (spectators) were found to accumulate on both Pt and support surfaces. It was found via the NO/H2/16O2 f NO/H2/18O2 isotopic switch that the reaction path from NO to form N2O passes through the oxidation step of NO to NO2 with the participation of gaseous O2, where the extent of it depends on support chemical composition.
Introduction The selective catalytic reduction (SCR) of NO by H2 in the presence of strongly oxidizing conditions (>5 vol %) has attracted attention in the recent years.1-12 This is mainly due to the imposed stringent regulations of NOx emissions from mobile and stationary sources with gas emission temperatures lower than 300 °C. It was first shown13,14 that H2 can reduce NOx at the lowest possible reaction temperatures. Today’s great concern about the increasing emissions of carbon dioxide to the atmosphere will require the use of appropriate non-carboncontaining reducing chemical species for the development of new lean de-NOx catalytic technologies.15-17 In addition, the facing problems of the currently used NH3-SCR process in stationary power sources and chemical plants,18 e.g., the use of separate ammonia source and injection system, ammonia slip, toxicity and corrosiveness, and operational cost make desirable the replacement of ammonia with another reducing agent. Hydrogen -SCR could be considered as such an attractive alternative for the NH3-SCR process. For many conventional catalysts, the NO/H2 reaction is strongly inhibited by oxygen due to the competition between adsorbed NOx and oxygen species for activated adsorbed hydrogen species. Platinum supported on γ-Al2O3, SiO2, and TiO2 was found to be the most active for the NO/H2/O2 lean * To whom correspondence should be addressed. Tel: +35722-892776. Fax: +35722-892801. E-mail:
[email protected].
de-NOx reaction at temperatures lower than 200 °C compared to supported Rh and Pd catalysts.1-7 However, these catalytic systems present significantly low selectivity values toward N2 formation (SN2
